Could kelp forests keep ocean acidification at bay?

The ocean is absorbing some of the excess carbon dioxide humankind is emitting into the atmosphere. As a result, seawater is becoming more acidic, with profound repercussions for marine life across the food web.

Scientists theorize that by sucking up carbon dioxide from seawater just like land plants do from the air, kelp forests could help provide local refuges against ocean acidification.

A five-year project launching this winter aims to test this theory by planting a kelp forest in the waters of Washington’s Puget Sound and monitoring its effect on water chemistry.

Scientists have long argued that restoring forests on land could mitigate global climate change, but what about restoring forests in the sea? Not forests made up of trees, of course, but of kelp: giant, brown algae that thrive in shallow, temperate seas and provide habitat for numerous species.

Scientists theorize that by sucking up carbon dioxide from seawater just like land plants do from the air, kelp forests could help mitigate the other nasty side-effect of our fossil fuel addiction: ocean acidification. Scientists will test this theory this winter by launching a five-year large-scale kelp forest study in the waters of Washington’s Puget Sound.

“Kelp, seagrasses, mangroves, and marsh plants consume carbon dioxide in the process of photosynthesis. Scientists and others are now asking whether marine vegetation can consume enough carbon dioxide to make a meaningful difference in local seawater chemistry,” said Terrie Klinger, co-director of the Washington Ocean Acidification Center at the University of Washington.

Researchers examine bull kelp grown in a pilot planting in preparation for a study on the effect of kelp forests on seawater acidity in Washington state’s Puget Sound. Photo courtesy of the Puget Sound Restoration Fund

This winter a local NGO, the Puget Sound Restoration Fund, plans to begin testing how effective kelp actually is at mitigating ocean acidification by growing forests from scratch. The Washington Ocean Acidification Center is a partner in the experiment currently under preparation in Washington state’s Hood Canal, the westernmost section of Puget Sound.

Both climate change and ocean acidification stem from the same problem: a massive influx of carbon dioxide into the atmosphere due mainly to people burning fossil fuels and slashing terrestrial forests.

Less familiar to the public than climate change, ocean acidification means that the pH level of marine water is dropping — acidifying — as the ocean absorbs excess carbon dioxide from the atmosphere. To date, the average pH of the ocean has fallen from 8.2 to 8.1. But that seemingly small drop brings a big impact.

When seawater becomes more acidic, many species — adapted over eons to historical conditions — suffer. For example, greater acidity means less calcium carbonate available in the water for calcifying organisms, from shellfish to corals and plankton to sea urchins.

“We know that oyster larvae have a harder time developing at conditions that we find in our waters now,” Jan Newton, the other co-director of The Washington Ocean Acidification Center, told Mongabay. “We see that pteropods, a fragile plankton that is part of the food web, have visible dissolution in their shells…some even with holes.”

If the ocean acidifies too much and organisms at the bottom of the food web crash, scientists worry the effects could ripple to the very top, disrupting fisheries that millions of people depend on.

The waters of the U.S. west coast are already naturally more acidic than many other places in the world due to the upwelling of carbon-saturated water from the ocean’s depths. Water in Puget Sound sometimes plunges to a pH of 7.8, which makes it a good testing ground for what the ocean of the future might look like. (Scientists expect that the average global pH level could fall to 7.7 by the end of the century given our current rate of carbon emission.) And also a good testing ground to see if kelp could actually raise pH back to a more normal level in the water around it. If so, kelp forests could provide safety nets for animals in a time of rapid ecological change — a local reprieve from a global problem.

Since kelp is one of the fastest growing organisms in the world, the Puget Sound Restoration Fund project will have to move quickly to keep up. The team plans to establish seedlings, potentially of two different species — bull kelp (Nereocystis luetkeana) and sugar kelp (Saccarina latissima) — over one hectare of Hood Canal in December of this year. Team members will string the seedlings between buoyed rafts. The kelp will grow downward into the top three meters of the shallow water, creating an upside-down forest.

Once winter is over, the kelp will start growing amazingly quickly, up to 18 inches a day. This is when more than a dozen scientists will begin gathering constant data on how the marine forest is impacting the water’s chemistry both inside and outside the kelp rafts. It’s especially important to the team to collect data both during the day, when the kelp takes in carbon dioxide as it photosynthesizes, and at night, when it releases some of the gas.

As spring turns to summer, the kelp will reach a length of several meters. Then, next May or June, the researchers plan to take the kelp out.

“The key is to take the CO2 up, and then to remove the kelp. If not, as soon as it degrades (just like your compost pile) then CO2 is released back again to the water, through the respiration of the bacteria breaking down the kelp,” Newton wrote in an email. The species in the study live for just one year.

The team members haven’t yet decided what to do with the kelp once they harvest it, but possibilities include selling it as food, compost, or even biofuel. Scientists have long proposed kelp and other marine algae as having huge biofuel potential given its remarkable growth rate and the fact that kelp farms wouldn’t compete with food for arable land and could provide habitat for marine species during much of the year.

Of course, this means that the carbon sequestered by the kelp will eventually be released back into the atmosphere. But at least it will be removed from the oceans, where it is causing ocean acidification. And if it is converted to biofuel, it may also eliminate the need for some new emissions from burning fossil fuels.

The kelp will also help remove nutrient pollution that runs into the sea from agricultural land, according to oceanographer Simone Alin with the National Oceanic and Atmospheric Administration, another member of the project.

Alin noted that more research is needed to fully understand the role kelp forests play in the carbon cycle, but it’s likely that established ones are carbon neutral Unlike trees, which store the carbon they take out of the atmosphere for the duration of their decades- or centuries-long lives and decompose slowly, kelp lives anywhere between a year and seven years, and breaks down relatively quickly.

With a $1.6 million grant from The Paul G. Allen Family Foundation, the project will continue through a second crop rotation before analyzing its data in 2019.

The team doesn’t just expect that kelp forests will help mitigate ocean acidification in nearby waters.

“There may well be fisheries benefits in the vicinity of and within the kelp field,” Joth Davis, lead scientist at the Puget Sound Restoration Fund, told Mongabay. “We know that we will be creating a substantial amount of seaweed habitat that will be certainly colonized and utilized by fishes and mobile invertebrates.”

Davis said various institutions — the Port Gamble S’Klallam Tribe, the Washington Department of Natural Resources, and the Washington Department of Fish and Wildlife — plan to survey the kelp forest in the spring to see which animals are using it.

Whatever the outcomes of the research, within a few years scientists and policymakers will have a much better understanding of just how strong a safety net kelp forests could provide for marine species in a time of ecological upheaval.

Kelp and sardines in California’s Channel Islands National Marine Sanctuary. Photo courtesy of NOAA’s National Ocean Service